this is a known issue. i'm not so sure but i believe you boot with something like "rd=disk0" or something, google it pretty sure you'll find it. check out this post Possible solutions for still waiting-for root device

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CPU: Intel Pentium Dual E2140 1.6 GHz. Graphics: Intel(R) 82945G (128 MB). Board: Asus P5GC-TVM/S Chipset: Intel Lakeport-G i945GC. HDD: 232 Gb S-ATA Samsung
Version 10.6.2 32bit. Kernel: Vanilla. Audio: VoodooHDA 0.2.2 with prefpane. Video: Stock GMA. PS2 fix: VoodooPS2
USB 2 not working

Acer Aspire 4530-6823
CPU: AMD Athlon™ X2 QL-62 2.0GHz. Graphics: Integrated GeForce® 9100M G. Chipset: nForce® MCP77MH.