Dear all!
I have encountered a really strange problem I cant figure out by googling, so I need to rely on your expertise.

Currently I am able to mount the formerly internal Harddisk as an USB drive with other computers using Mac, Windows or Linux Live CD. The partition table seems correct after first recognition of the drive by the system, and I can also access the mounted volume and see files and folders for a moment. It is even possible to open a file. If I try to open further files or browse the data structure for too long (which is between a several seconds to a few minutes) I get a Input/Output error (In case of Knoppix and Mac) and no action would work any more. Unmounting and mounting the device again results in an unkown RAW partition, but still with a known size. Disconnecting the drive lets me repeat above procedures. Recovering single small files is possible that way, whereas this will obviously fail for large files or even a whole partition.

History: After updating Windows and a subsequent bluescreen some hours after the next boot-up windows once restarted flawlassly but never revocered from the next hibernation. I thus tested several startup procedures including protected mode, repair (even from Recovery DVD) etc. always freezing at the step when the movable mouse appears upon a graphcal window. I finally booted into my Linux Mint partition which worked perfectly, and let me do some backups. Unfortunately only some, as the next day I booted the system even Grub 2 was gone. As the Windows Recovery DVD and Linux Mint Live couldn't see the harddisk, i built it into a external hard drive mount with USB2 and plugged it into my friends computers, namely a Mac OS X and a Windows Vista system, where I could at least see, that the data is still there and eventually accessible.

Harddisk (according lable):
Seagate Momentus
SATA
750GB
7200rpm
Firmware 0002DEM1
ST9750420AS

My computer:
Dell XPS 15 L502X
Intel Core i7-2679QM @ 2.20GHz
RAM 8GB
Original Windows 7 Home Premium 64bit SP1
One dell utility partition and one untouched recovery partition before the OS and extended partitions

As I do not have any raid system I generally backup my important Data manually just abut once per year (really important data is synced via cloud services), therefore many (at least to me) valuable files are still on the harddrive, so I would thankfully appreciate any hints or solutions. Could this be a hardware malfunction, a harddisk controller error a magnetic error or some filesystem and partition table corruption? Kind regards
mimesot

From what you have mentioned, the drive seems to have internal failure.
The chances are that the drive will completly fail soon and will stop detecting in the USB enclosure also .
Stop trying to access the data by mounting and remounting the drive everytime it fails to recognise as this will degrade the drive faster.
If data is not that important, RMA it.
If you have valuable data , worth the time and money , then :-
A DR company will be able to comfortably get back most of your data.It shouldn't cost you much either as this does not seem to be a major problem(assuming from what you have mentioned, cant be sure unless further tests are run).
If you want to DIY, you should fully understand the consequences
(chances are that it may not give you the desired result,
you may end up wasting your time and money(you need a wroking drive with a similar or larger capacity for the clone process))
You will have to clone the faulty dirve to a larger working drive and try to run recovery tools on that drive to get your data.
You will have to connect the drive directly to the SATA port and not in the USB enclosure and use dd_rescue.
good luck

@OP practice first the dd-rescue command. Easy to make mistake and ruin any chance at recovering data. Of course if this option is feasible for you.

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Hard Disk Drive (HDD), Solid State Drive (SSD, SATA, NVMe, etc), USB Flash Drive and RAID Data Recovery Specialist in Massachusetts

Thank you both for your replies. ddrescue is not dd_rescue as i can see but to which one do you refer labtec? I really like to learn new things as it is surely practical, but how do you mean practicing? Something like manually de-partitionate a testing device and recover it? Sathyan, by RMA you mean sending it back to the manufacturer, right? I will dig into the rescue mechanism to understand the risks better (under the assumtion of correct operation) as I have no idea what it actually does. Again Thank you very much!

_________________
Hard Disk Drive (HDD), Solid State Drive (SSD, SATA, NVMe, etc), USB Flash Drive and RAID Data Recovery Specialist in Massachusetts

There are some tools like HDD raw copy/MTP which do a bit by bit copy from source to target drives.
Some have the option to clone in reverse too(start clone from the end of the drive ) which gives better /different results depending upon the problems.
If you are familiar with linux, then ddrescue is the option for you as it is free tool.
Others like MTP are not free.
Good luck