How old's the drive?
There's basically three likely causes:
1. faulty cable - test with another cable from one of the other drives
2. faulty interface bus in the drives case - swap the drive into another enclosure and test
3. physical wear on the drive platters - time to buy a new disk.
Regardless, any data on this disk should be backed up immediately.
I'm not sure if yours is the same model, but there's a world of painful stories from people having similar problems with Lacie drives in this thread.
There are also a variety of solutions offered, some worked for some and not for others. Might give you some ideas for how to more forward (p.s., if its a double drive RAID unit it doesn't look like you can swap the enclosure anyway, so forget my no.2).
I reformatted the first one and so far so good. After 1 week of copying files to another disk. I hope it holds out.
All of a sudden..... egads.... Another one just went out. I have 3 Lacie 1TB drives.
This last one will not even come on. Any suggestions? I have (had) tons of family videos on it. But I know I had at least 300 GB left on it.