Can i stack my nas (ds918+) directly on top of the expansion unit? physically i mean.

Mine is stacked on top. I haven't had any issues. I have had it for about a year.

Why expansions has the same price that normal units? is pointless buy expansions.

I don't know where the price comparison is that says the expansion chassis is the same price as a stand-alone unit. If you're comparing Synology to Synology, the only 5-bay NAS in their current lineup (2019) is the DS1019+ and that's almost $200 more than the DX517. That ~$200 buys you the flexibility to run an entirely separate NAS in a different location. It also means two different systems to maintain and keep updated. But if you already have a NAS that can accept that the expansion chassis has to be right next to your main NAS, it's still more cost effective to go this route. You can still set up separate volumes with different sized HDs if you wanted to. Or you can pool as many drives as can fit into the expansion chassis into your existing volume.

Can i cluster the drives - eg. 2+2+1, meaning 1st 2 are 2 hdd in raid, 3rd & 4rd are 2 further hdds in raid for different content, 5th is single?

Yes. The expansion box just adds up to 5 more drives to the DSM. You can allocate those drives to any volume you create. They can be new volumes clustered as you have described or existing volumes already on the server.

I was wondering what is better connecting the dx517 to the ds918+ like the user below did or just spend a little more & get the ds1817+?

Depends what you want to do with it. If all you need is massive cloud storage - photos. Music, etc., or for security camera footage storage, the DS1817 is a great choice. If you want video transcoding on the fly, though, everything seems to point to the 919, which has that software built in, while the 1817 does not. In either event, spend a little extra and get the dual cache chips and turbocharge your selection.

Brand	Synology
Product Dimensions	24.79 x 23.29 x 15.7 cm; 3.91 kg
Batteries	1 Lithium Ion batteries required.
Item model number	DX517
Manufacturer	Synology
Series	DX517
Colour	Black
Form Factor	3.5-inch
Standing screen display size	75
Processor Brand	Intel
Processor Type	5X86
Processor Speed	3.4
Processor Socket	BGA 413
Processor Count	1
Memory Technology	DDR3, DDR3, DDR3
Computer Memory Type	SDRAM
Maximum Memory Supported	8 GB
Hard Drive Size	80
Hard Disk Description	No Hard-disk
Hard Drive Interface	ATA-4
Hard Disk Rotational Speed	7200
Graphics Card Description	Dedicated
Graphics RAM Type	72-Pin EDO SIMM Memory
Graphics Card Ram Size	8
Graphics Card Interface	AGP
Number of USB 2.0 Ports	1
Number of USB 3.0 Ports	1
Voltage	1 Volts
Wattage	29.9 watts
Optical Drive Type	BD-R
Hardware Platform	Windows, MAC, Linux
Operating System	Windows 11
Are Batteries Included	No
Lithium Battery Packaging	Batteries only
Lithium Battery Weight	0.19 Grams
Number Of Lithium Ion Cells	1
Number of Lithium Metal Cells	1
Item Weight	3.91 kg
Guaranteed software updates until	unknown

Synology DX517 5 Bay Desktop Network Attached Storage Expansion Enclosure, Black

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