If it's not formatting to the same capacity then it might not be an exact replacement. Over time manfacturers change the internals of the drives and so even if it has the same name and size on the label, might be very different on the inside.
When you do an "info log t" in the command line, what model and firmware revision does it report for all the drives? For example, I had four Western Digital drives in my 705N, they were all WD1200JB, and when one of them went out I bought a new one as a replacement. The label has the same model and drive size, but when I check the info in the log it reports:
Code:
10/07/2006 15:13:49 45 D SYS | Intf: 0, dev: 0: Model: WDC WD1200JB-00REA0
10/07/2006 15:13:49 45 D SYS | Firmware Rev: 20.00K20 Serial #: WD-WMANN1132794
10/07/2006 15:13:49 45 D SYS | Intf: 1, dev: 0: Model: WDC WD1200JB-75CRA0
10/07/2006 15:13:49 45 D SYS | Firmware Rev: 16.06V16 Serial #: WD-WMA8C1305039
10/07/2006 15:13:49 45 D SYS | Intf: 2, dev: 0: Model: WDC WD1200JB-75CRA0
10/07/2006 15:13:49 45 D SYS | Firmware Rev: 16.06V16 Serial #: WD-WMA8C1309021
10/07/2006 15:13:49 45 D SYS | Intf: 3, dev: 0: Model: WDC WD1200JB-75CRA0
10/07/2006 15:13:49 45 D SYS | Firmware Rev: 16.06V16 Serial #: WD-WMA8C1321357
Drive 1 is reporting a different firmware rev and when I format it, it does not come up with the same capacity as the older drives. But in my case it shows more, not less like yours. Unfortunately, even in this case I couldn't get it to rebuild the raid. At the moment I've backed it up to another 705N and shut it down until I have some time to play with it some more.
All I can suggest is that you may want to get a drive with the next step up in size, like maybe an 80GB and see if that works.