I just inherited a used T21. I opened it up and see that it has a 20GB HD. I'd like to replace that with a 120GB HD. The userguide shows a maximum 60GB HD from IBM. Is this the maximum size HD that a T21 will take? If not, what 3rd party HDs will work? I see that the Toshiba MK1234GAX - hard drive - 120 GB - ATA-100 has gotten good reviews. Will that work? If not, will any 120GB HD?
Also, I have not tried to fire it up yet, but am guessing that I may have a CMOS battery problem. Where can I buy replacement CMOS batteries? Are these model specific? Thinkpad brand specific? Laptop category specific? Or totally generic?

All CMOS batteries I've seen are just standard CR2032 batteries which EVERYONE carries. Far as the hard drive, you should be able to run the 120GB with no issues.

Ahhh ... the T21 ... had a T20, 21 and 23 in the past, great laptops. It will definitely take 120GB ... I'm trying desperately to remember if I had a 160gb in a T2x.

I also think from memory it can actually take 12.5mm drives, but avoid that - the drive is directly below the wrist rest and I broke one by putting too much pressure on. The 9.5mm drives (2 platters like everything upto 320GB) today are well clear so no probs.

I'd have a good read of the update info for the BIOS updates on the Lenovo thinkpad site ... it's quite possible that it will take more that 120GB ... the other possibility is to risk getting a 160 / 250GB as the cost difference is minimal. If it isn't compatible with big LBA then you'll just get 137GB capacity usable on those drives (versus about 111 from a 120) ... it might be worth the extra £10 / $20 to get that 26GB anyway??

Good luck!