FortiGate Next Generation Firewall utilizes purpose-built security processors and threat intelligence security services from FortiGuard labs to deliver top-rated protection and high performance, including encrypted traffic.
Article Id 195969
'get system status' output shows 'Log hard disk: Not available' when the physical hard disk is present in the Unit
This could cause issue while introducing the RMA unit or introducing the unit back in cluster after factory reset or flash format as the Hardware is same between both the units

All FortiGates with HDD.

If the FortiGate has one hard disk, then it can be used for either disk logging or WAN optimization, but not both.
By default, the hard disk is used for disk logging.

When the hard disk is being used for WAN optimization, it displays 'Log hard disk: Not available' in the get system output.
# get sys status
Version: FortiGate-3000D v6.0.10,build0365,200617 (GA)
Virus-DB: 1.00000(2018-04-09 18:07)
Extended DB: 1.00000(2018-04-09 18:07)
Extreme DB: 1.00000(2018-04-09 18:07)
IPS-DB: 6.00741(2015-12-01 02:30)
IPS-ETDB: 6.00741(2015-12-01 02:30)
APP-DB: 6.00741(2015-12-01 02:30)
INDUSTRIAL-DB: 6.00741(2015-12-01 02:30)
Serial-Number: FGT3KD3Z16800075
IPS Malicious URL Database: 1.00001(2015-01-01 01:01)
Botnet DB: 1.00000(2012-05-28 22:51)
BIOS version: 05000004
System Part-Number: P17065-02
Log hard disk: Not available
Hostname: FGT3KD-1
Operation Mode: NAT
Current virtual domain: root
Max number of virtual domains: 10
Virtual domains status: 1 in NAT mode, 0 in TP mode
Virtual domain configuration: disable
FIPS-CC mode: disable
Current HA mode: standalone
Branch point: 0365
Release Version Information: GA
FortiOS x86-64: Yes
System time: Wed Sep 23 09:31:42 2020

# get hardware status
Model name: FortiGate-3000D
ASIC version: CP8
CPU: Intel(R) Xeon(R) CPU E5-2650 v3 @ 2.30GHz
Number of CPUs: 40
RAM: 64469 MB
Compact Flash: 15331 MB /dev/sdb
Hard disk: not available
USB Flash: not available
Network Card chipset: Intel(R) Gigabit Ethernet Network Driver (rev.0003)
Network Card chipset: FortiASIC NP6 Adapter (rev.)
This prevents the units in forming HA cluster as the hardware is not same in this case.
Existing unit in cluster would have 'Log hard disk: Not available' and the factory reset or RMA unit will have'“Log hard disk: Available'.

To confirm if the HDD is being used for WAN optimization, check using below command.
# diagnose hardware deviceinfo disk
  Disk HDD1            ref: 255 447.1GiB    type: SSD [ATA ADATA SX1000L] dev: /dev/sda
  partition ref:   1 125.4GiB, 124.9GiB free  mounted: Y  label: WANOPTXX02F8ADD2 dev: /dev/sda1 start: 2048
  partition ref:   2 127.4GiB, 127.4GiB free  mounted: N  label:  dev: /dev/sda2 start: 267245568
  partition ref:   3 169.9GiB, 169.9GiB free  mounted: N  label:  dev: /dev/sda3 start: 534489088
To form HA between the peers, configure HDD for WAN optimization on the RMA or factory reset unit.
# config system storage
       edit "HDD1"
          set usage wanopt
Above command will make the HDD dedicated for WAN optimization and the disk memory will no longer be available for the logging.
Now the hardware will be similar and the HA shall be formed between the peers and allows to sync the configuration from the Master unit.

Related document.