Some research shows that these drives have a common failure in the controller board, trying to figure out if I'm on the right track before I waste money on a replacement board or just scrap the drive and the data.

If I power cycle the drive, it works fine briefly, before throwing out input/output errors, and then finally not being recognized by my Arch Linux box. Process repeats if I power cycle the drive. I haven't heard any weird clicking behaviour from the drive (at least that I've noticed)

What's your vote, drive or controller? Thank you!

Probably some media issues (bad sectors)

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PC Image Data Recovery
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not a controller problem for sure.
it could be bad sectors, FW or even 1/2 weak heads.

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Data Recovery Pakistan

IMO dont waste money on the PCB. Unlikely to be the culprit. You have a bit if functionality - check SMART.
My books depending on year of manufacturing use different drive families. Saddle G6 notorious for losing head(s). Dragonfly and Diablo mostly service area problems.

Thanks for the advice!

Since the drive is half alive (at least for periods of time), any suggestions for how to get some of the data off it?

Try ddrescue. Since this is probably a one-off chance, consider using the services of a local Data Recovery Lab to ensure your data's safety.

Agree, if it's no big deal if you lose the data then give ddrescue a shot to clone the drive to a known good one,

If the data is important then better to secure the services of a DR lab, it shouldn't be expensive at this stage.

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PC Image Data Recovery
http://www.pcimage.co.uk

New!! HDD-PCB.COM for all your PCB and donor HDD requirements!

If your data is worth my minimum price of $350 for a 2TB drive and you can get it to Guelph, I'd be happy to look at it for you.

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Luke
Recovery Force Donor Inventory: 5,000+ Fully Cataloged HDDs & Growing

At the very least, examine the SMART report with smartmontools or similar. This will confirm whether the drive itself has physical problems or whether there is a problem with the enclosure or cabling (unlikely).

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A backup a day keeps DR away.