Had the problem last week to. Also RAID 5 using 5 disks. I also accidentaly removed one disk when the systen was swithced on.
I solved the problem by:
At first stay of all your other disk from now on!!!!!!
shut down the Promise system
Remove the dead disk. Remove the disk from the slide (4 mounts see manual)
Format the disk on an external disk system (in the Netherlands about 35 Euro). You cannot use the Promise box because he regognises the disk as dead. But dont forget also to renew the partition on the disk. Just formatting is not emough. Promise recognised my disk still after simple formatting. This is also the point were you can see wethher the disk is still usable, In my case the disk was still OK.
After the new partioning and formatting The disk is seen as a new disk by promise. put ik back in the slide en put in in de promise system.
now in the utility App ; make from the disk an " unconfigured" one. An than in the utility App "rebuild" and he will use the formated disk for rebuilding your RAID.
At least it did so in my case. I sleep well after soving this problem, I can tell you.