Ok, at the "DVD EA"-screen (step 7) press x and wait untill the disc spins down. Then u Swap in your backup (using fliptop, slidekey or other way) without the ps2 knowning - the "media invalid" is a message telling us that your ps2 has discovered that u have inserted a new disc (and it should not have done that) - so the swap didnt go well (most likely).
Once u manage to sneak that backup into the ps2 u press x and it should boot.
Maybe your media is incompatible with the ps2, some media is, so try a different brand. Event tho u can play backup movies on your media it might not work with games. Any type of RW media will not work.
Try making your backup using dvddecrypter (from www.dvddecrypter.com) - it make good Ps2 backups.
PSP 1.52 (DG), DS 001, XBOX v1.6B+Cheapmod+120GBHD, SM33, 50004+HDA10+120GBHD, SM32, BP11, 3900X, XBOX v1.6+Softmod, P4_2.666, GBASP, GBA,Celeron_700, DC, PS1, P2_300, TI85, Saturn, GBC, GBP, Megadrive, P75, Amiga 500, MS, 486SX25, Amstrad CPC464, ZX81