As you may know the MD253 runs an Oxnas 810 SoC. The Iomega Home Media Network Hard Drive
(I didn't invent that name) uses the same SoC. A big difference is that the MD253 boots from flashrom, while the HMNHD has no flashrom at all, it boots directly from harddisk.
Oxnas is called PLX now. The successor of the HMNHD, the HMNHD CE
runs the successor of the Oxnas 810, the PLX 7820, and uses the same trick, booting directly from harddisk.
Medion sells a Nas, the P89626, which also runs a PLX 7820, but it boots from flashrom, just like the MD253.
Now I'm coming to my point: According to this German wiki
the PLX 7820 can boot from Sata, SPI and flashrom. In this sequence
. So by putting a well-prepared disk in a P89626, it will boot from Sata, instead of flashrom.
When the PLX 7820 behaves this way, maybe it's predecessor, the Oxnas 810 does the same. In that case you could prepare a disk using this directions
, and boot the MD253 from it. When that succeeds, it's relatively easy to exchange the Iomega firmware by Debian Lenny. More here
. The described rootfs can also be put on the harddisk.