I don't think this system is M.2/SSD - compatible. The problem with M.2 expansion slots is that it allows to use connector TYPE, not necessarily provide COMPATIBILITY. That is: depending on the implementation of the system board manufacturer it can be all, any or either of PCIe, SATA or USB. All depends on the wiring and components.
There is nothing in the documentation on the E5550 that it's M.2 is SATA compatible. Sorry. In fact, nothing says anything on compatibility of those slots, so it's safe to assume it's only PCIe/USB or just USB compatible. But you'll never know, so best bet is to try first - search for friendly repair shop and ask them to fit one mSATA drive to test it works.
EDIT: M.2 support of SSD depends on how the slot has been implemented on the specific motherboard. If pins are wired according to M.2/SATA standard, then all is good - M.2 SSD will be connected to onboard SATA controller. However, it obviously requires paths being laid on the motherboard between M.2 slot and SATA controller. If M.2 is implemented as PCIe-only, it will be connected to chipset (simplification), if as USB - then it will connect to USB hub. If as full M.2 (all-compatible), then it will accept any device in proper factor and connector-type. But it's most expensive to do (as it requires extra hardware), so it's rare in laptops, especially low-mid range.
EDIT2: M.2 is a new version of mSATA. Sort of... There is physical connector difference, for starters. But: mSATA is an essentially SATA-compatible drive with connector of the size and format of mPCIe slot. it differs from mPCIe in pin arrangements. So, in essence, mSATA looks like mPCIe but it's not, however if motherboard manufacturer so chooses, the slot will support either (or more ;). Usually manufacturers both mark and market given motherboard/system as having mSATA connector. Same with M.2 - it needs to be manufactured as supporting SSD drives and usually will be marked as such.
Other difference is that M.2 is not limited to connecting to SATA controller (like mSATA was). It can use PCIe bus, which is waaay faster.
I hope answer is now clearer, as my mental shortcutting in the original version could be very confusing.