From LedHed's Wiki
You can tag an automatically detected SSD device as a non-SSD device.
Procedure
1. Identify the device that you want to untag.
esxcli storage nmp device list
You might see the output similar to the following:
naa.6006016015301d00167ce6e2ddb3de11
Device Display Name: DGC Fibre Channel Disk (naa.6006016015301d00167ce6e2ddb3de11)
Storage Array Type: VMW_SATP_CX
Storage Array Type Device Config: {navireg ipfilter}
Path Selection Policy: VMW_PSP_MRU
Path Selection Policy Device Config: Current Path=vmhba4:C0:T0:L25
Working Paths: vmhba4:C0:T0:L25
2. Note the SATP associated with the device.
3. Add a claim rule with the option disable_ssd.
esxcli storage nmp satp rule add -s SATP --device device_name --option disable_ssd
For example,
esxcli storage nmp satp rule add -s VMW_SATP_CX --device naa.6006016042fa19010a12d9b16d6ade11 --option disable_ssd
4. Reclaim the device.
esxcli storage core claiming reclaim --device naa.6006016042fa19010a12d9b16d6ade11
5. Check the device status by running the following command.
esxcli storage core device list -d device_name
The command output indicates whether the disk is untagged.
Is SSD: false
Notes
I came across this after upgrading my NAS. Every new iSCSI target I created was being detected as SSD by ESXi, but they weren't SSD disks.