1) I have Seagate Freeagent 1.5TB External HDD. I use Windows 7.

I transfer data from External HDD to my computer.

Although there is a lot of space on my computer progress bar turns red after it transfered some files and ex. hdd doesn't respond.

What's the problem? How to fix it?

2) Is there a life span of external HDD's? Should I renew of my external HDD's and dump my old ex HDDs?

Not surprising to hear on that model drive.

Check SMART info for starters.

Always create backup if the device gives sign of failing.

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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