I've tried TDK, Verbatim and JVC without success, there must be a brand that works with my iMac! The drive I use is as follows:

MATSHITA DVD-R UJ-875:

Firmware Revision: DB09
Interconnect: ATAPI
Burn Support: Yes (Apple Shipping Drive)
Cache: 2048 KB
Reads DVD: Yes
CD-Write: -R, -RW
DVD-Write: -R, -R DL, -RW, +R, +R DL, +RW
Write Strategies: CD-TAO, CD-SAO, DVD-DAO
Media: Insert media and refresh to show available burn speeds
Thanks in advance!
Cheers,
DB