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.
But I got that reply that it supports
Solid-State Storage Devices
. But I will not try it because it is not very reliable info - maybe it is about M.2 in general. – Vojtěch Dohnal – 2015-05-05T13:02:18.620SSD comes in various formats. It definitely supports SATA SSD (e5550 that is). If we're talking about M2 - then it supports SSD as an INDUSTRY STANDARD, depending on implementation. Pretty much any SATA system can do that, though. tricky part begins with mSATA slot. AS you answered yourself, you can buy adapters and fit mSATA in the SATA drive slot using system-included cables/connectors. – AcePL – 2015-05-05T13:06:04.060
So theese M.2 SSD disks are mSATA disks only with different connector? – Vojtěch Dohnal – 2015-05-05T13:35:15.703
I wanted to add SSD disk as a second drive. – Vojtěch Dohnal – 2015-05-05T13:37:50.043
If you want to add SSD as a second in addition to already installed hard drive then mSATA would be one option. Second is to buy special caddy that fits into the DVD bay (but, obviously, you will have no optical drive in the system). Personally, I'd recommend that. You can at the same time buy USB adapter for your ODD and use it as external DVD. That is my configuration ATT. I found out that I needed DVD exactly three times last two years. – AcePL – 2015-05-05T13:53:32.173
Oh, missed your other comment. M.2 is a new version of mSATA. Sort of... 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. 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. – AcePL – 2015-05-05T15:09:11.300