Does anyone know if RouterOS can run on ASUS Eee PC?
In such case, is it possible to install it using a USB drive or a SD card?
Why would you do this? 
Just tried V3.10 on a USB stick in time-limited mode. It ran, but had no idea about the ethernet card. It knew it had to load the atheros driver for the atheros card, but no interface was available. Might need a patch on the wireless at least to use the AR5007 in these things. The Lan is an “Attansic Technology Corp L2 100Mbit Ethernet Adapter”. I suppose USB 2 Ethernet would get around that…
As to why one would want to? Battery backed up accesspoint complete with screen, keyboard and built in antenna with 512MB ram, 4GB SSD and 630/900Mhz processing power. Not a bad little ap, esp if you can get them cheap enough. With the SDHC card reader looking like a usb stick to the system one can even say, boot off a 32GB SD card (Only as an example.. 
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As the school I was working at is going to get 260 of these, would be good to use a usb key to boot into an accesspoint temporarily when one of our other mikrotik ones die (Deaf radios.. 
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Booting off the USB/SD won’t affect the onboard SSD, and I can make a script to configure the wireless card and/or ethernets to a bridge. Simply boot, run script, enjoy AP… 